Method for producing a high temperature-resistant target alloy, a device, an alloy and a corresponding component

ABSTRACT

The present invention relates to a method for producing a high temperature-resistant target alloy. The method comprises 
     (a) applying a vacuum to an attritor vessel containing the base material of the target alloy, 
     (b) filling the attritor vessel with a powder containing the base material of the target alloy with a reduced alloy element content, 
     (c) filling the attritor vessel with grinding balls containing the base material of the target alloy, 
     (d) rotating the agitator of the attritor and/or the attritor vessel. 
     The powder is alloyed by attrition of the attritor and/or attritor vessel and the grinding balls themselves. The invention also relates to a corresponding device for carrying out the method.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 of German Patent Application No.

102014222347.4 filed Nov. 3, 2014, the entire disclosure of which is expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for producing a high temperature-resistant target alloy, in particular a TiAl alloy. In addition, the invention relates to a corresponding device for carrying out the method, the corresponding alloy and use of the device for producing the high temperature-resistant target alloy.

2. Discussion of Background Information

For operation of continuous flow machines, due to the conditions of use of the components used, which are exposed in part to high temperatures, aggressive environments and high forces, special materials are required for certain components which are optimally conformed to the intended purpose both by their chemical composition and by their microstructure.

Alloys based on intermetallic titanium aluminide compounds (TiAl alloys) are used in the construction of continuous flow machines, such as stationary gas turbines or aircraft engines, for example as a material for rotor blades, since they have the mechanical properties necessary for the purpose and additionally have a low specific weight, such that the use of such alloys may increase the efficiency of stationary gas turbines and aircraft engines. There is accordingly already a plurality of TiAl alloys and methods for producing corresponding components therefrom.

Like comparable components made from other high temperature alloys, for example based on Ni, Fe or Co, components made from TiAl alloys may be manufactured both melt metallurgically and powder metallurgically.

In powder metallurgical production, the manufacturing steps comprise, in addition or as an alternative to the individual steps of melt metallurgical production, the use of powder materials to produce a desired composition of the material for example by alloying. An example of the production of an article from a TiAl alloy using powder materials is described in U.S. Pat. No. 5,424,027, the entire disclosure of which is incorporated by reference herein.

The powder may be produced for example from a molten bath, which is atomized by means of helium or argon at a very high cooling rate of up to 20,000 K/s. This results in a material with a microstructure which is intended to have a homogeneous and uniform grain structure. However, different particle sizes arise, which have to be separated out with effort by fractionation (for example by screening), such that to produce a component, powder may only be used which has powder particles of a specific minimum and a specific maximum diameter. Moreover, the powder must be subjected to multistage heat treatment, so as to optimize appropriately the micro-grain structure thereof This includes solution annealing, high temperature annealing and precipitation annealing. For this purpose, temperatures of over 1000° C. are needed for several hours. With these heat treatments care must be taken to ensure that no oxygen can reach the powder to be annealed.

In melt metallurgical production, the alloy used for production of the component is provided in the form of a melt and the latter is cast in a mold. The cast material has conventionally to be subjected to suitable forming and/or heat treatment to destroy the cast structure and establish a desired microstructure for the material. The corresponding component may then be brought into the desired shape by suitable finishing, for example by machining or mechanical or electrochemical processing. Segregation problems and coarse oxide particle inclusions occur in melt metallurgical processes in the case of high-alloy TiAl, Fe and Mo alloys. Segregation is to be understood to mean demixing processes in a melt. This results in the concentration of certain elements in a mixed crystal increasing at one point and the concentration of these elements decreasing at another point. This reduces the creep strength of the alloy at elevated temperatures.

In view of the foregoing, it would be advantageous to have available a method and a corresponding device for producing a high temperature alloy which on the one hand improves the creep properties and high temperature resistance of the high temperature alloy and significantly reduces or prevents contamination of the high temperature alloy by undesired elements.

SUMMARY OF THE INVENTION

The present invention provides a method for producing a high temperature-resistant target alloy, a device for mechanically alloying a high temperature-resistant target alloy, and a high temperature-resistant alloy as set forth in the appended claims.

In particular, the invention provides a method for producing a high temperature-resistant target alloy, comprising:

-   -   (a) applying a vacuum to an attritor vessel containing the base         material of the target alloy,     -   (b) filling the attritor vessel with a powder containing the         base material of the target alloy with a reduced alloy element         content,     -   (c) filling the attritor vessel with grinding balls containing         the base material of the target alloy,     -   (d) rotating the agitator of the attritor and/or the attritor         vessel.

According to the invention, the powder is here alloyed by attrition of the attritor and/or attritor vessel and the grinding balls themselves.

The components of the attritor include in particular an attritor vessel, a plurality of grinding balls and/or the agitator with a plurality of grinding arms. Through agitation, the grinding balls are hurled around in the attritor vessel and in the process strike the internal walls of said attritor vessel. Parts of the powder are then to be found between the surface of the grinding balls and the internal wall of the attritor. Components of the surface or the internal wall may then detach and in this way enter the atomic lattice structure of the base material. This has the advantage that the alloying components do not have to be present in powder form, which would enlarge the surface area of the alloying components. The alloying components would then form metal oxides to a greater and uncontrolled extent.

In one advantageous embodiment of the invention, rotation proceeds at a rotational speed of from about 30 rpm to about 300 rpm for a period of about 1 h to about 10 h. The duration and rotational speed depend on the size of the attritor vessel, on the quantity of powder in the attritor vessel, on the initial size of the powder particles prior to mechanical alloying and on the desired final size of the powder particles after mechanical alloying. In this respect, the final size is smaller than the initial size (here in terms of diameter), since the particles become ever smaller over time as a result of rubbing against the balls and against the other attritor components.

In a further advantageous embodiment of the invention, the powder is heat-treated, in particular by laser or electron beam melting and/or by hot isostatic pressing, in such a way that fine oxides, with in particular a size of about 1 to about 500 nm, are eliminated and/or the residual oxygen is gettered out of the crystal lattice of the powder. To this end, metals are preferably introduced atomically into the crystal lattice as alloying components through the mechanical work. The metals include transition metals and lanthanoids (rare earth metals). These atomic metals have a high oxidation capacity, such that, in the presence of sufficient excitation energy, these atomic metals bind the residual oxygen in the crystal to themselves and thereby form corresponding metal oxides. Binding of the residual oxygen is known as gettering (from the verb “to get”). The ductility, high temperature resistance and creep strength of the target alloy are thereby increased significantly. The objective when forming metal oxide particles is in the process to keep these particles small in diameter and to distribute them uniformly in the material matrix, in order in this way to achieve fine distribution of the metal oxides. The oxide particles may thus be used purposefully as ODS elements (ODS—oxide particle strengthening).

In a further advantageous embodiment of the invention, hot isostatic pressing takes place in a temperature range of from about 1000° C. to about 1500° C. for a period of about 1 h to about 10 h at a pressure of about 10 MPa to about 500 MPa. The duration, temperature and pressure depend on the desired degree of fine distribution and on the desired diameter of the metal oxides.

In a further advantageous embodiment of the invention, the powder of the base material comprises powder grains with a diameter of less than or equal to about 500 μm. The diameter of the powder grains is preferably greater than or equal to about 45 μm. This has the advantage that the powder of the base material with a greater powder grain diameter is less sensitive to undesired oxygen take-up.

In a further advantageous embodiment of the invention, the base material powder is plasma-cleaned prior to filling and/or at least one of the components of the attritor is plasma-cleaned prior to application of a vacuum. Preferably, degassing of the attritor takes place at a vacuum of from about 0.01 Pa (10⁻⁴ mbar) to about 0.1 Pa (10⁻³ mbar) for a period of from about 0.5 h to about 5 h and at a temperature in a range less than or equal to about 400° C. This has the advantage that the oxygen contamination of the alloying components and/or of the base material may be reduced or eliminated. In addition, this cleaning makes it possible to reduce or eliminate organic and/or inorganic impurities.

In a further advantageous embodiment of the invention, at least one of the elements Si, Y, Hf, Er, Gd, B, C, Zr, Y, Hf, Nb, Mo, W, Co, Cr, V is contained as an alloying component. Atomic yttrium, atomic hafnium and/or atomic zirconium form with the (residual) oxygen high temperature-resistant oxides, which pin down the lattice dislocations in the metal matrix and in this way improve creep strength at elevated temperatures (even at above 780° C.). Atomic erbium and/or atomic gadolinium likewise form oxides which improve oxide resistance. This means improved corrosion resistance of the target alloy with regard to oxygen. All the metal oxides listed are finely distributed by mechanical alloying without forming coarse oxide particles in the process.

In a further advantageous embodiment of the invention, at least one of the compounds from the group tungsten carbide, titanium-zirconium-molybdenum and hafnium-zirconium-carbon-molybdenum alloys and zirconium oxide, in particular stabilized with Y₂O₃, is included as an alloying component. For example, tungsten carbide is used to make the target alloy correspondingly harder.

In a further advantageous embodiment of the invention, those alloying components to be mechanically alloyed are present in the base material powder which may also be present in a proportion of over 0.5 at % in the target alloy. Alternatively or in combination, the powder of the base material may also comprise alloying components which are present in the target alloy in a proportion equal to 0.5 at %. This is advantageous because the accuracy of large quantities of alloying components greater than or equal to 0.5 at % can be better established in the base material than by means of subsequent mechanical alloying. The alloying components present in small quantities of less than or equal to 0.5 at % are preferably added by the mechanical alloying.

The powder of the base material preferably contains, in addition to the main constituents, in particular Ti and Al, the following elements in the stated proportions and is—apart from unavoidable impurities—formed from these: W: 0 to 8 at. %, C: 0 to 0.6 at. %, Zr: 0 to 6 at. %, B: 0 to 0.2 at. %, Nb: 4 to 25 at. %, Mo: 1 to 10 at. %, Co: 0.1 to 10 at. %, Cr: 0.5 to 3 at. % and/or V: 0.5 to 10 at. %. The values and numbers therebetween and not explicitly stated are also included.

The target alloy preferably contains, in addition to the main constituents, in particular Ti and Al, the following elements in the stated proportions and is preferably—apart from unavoidable impurities—formed therefrom: W: 0 to 8 at. %, Si: 0.2 to 0.35 at. %, C: 0 to 0.6 at. %, Zr: 0 to 6 at. %, Y: 0 to 1.5 at. %, Hf: 0 to 1.5 at. %, Er: 0 to 0.5 at. %, Gd: 0 to 0.5 at. %, B: 0 to 0.2 at. %, Nb: 4 to 25 at. %, Mo: 1 to 10 at. %, Co: 0.1 to 10 at. %, Cr: 0.5 to 3 at. % and/or V: 0.5 to 10 at. % The values and numbers therebetween and not explicitly stated are also included.

The invention further relates to a device for mechanically alloying a high temperature-resistant target alloy, comprising an attritor vessel, an agitator and at least one grinding ball. At least one of the components of the attritor coming into contact with a base material powder contains or consists of the base material and/or at least one of the alloying components of the target alloy.

The regions of the components which come into contact with the base material powder—apart from unavoidable impurities—preferably contain just one of the alloying components of the target alloy in addition to the base material. This prevents other undesired elements from the alloy composition of the components from being alloyed into the base material powder at an atomic level and thus contaminating the target alloy. The components of the attritor include in particular an attritor vessel, a plurality of grinding balls and/or the agitator with a plurality of grinding arms. This offers the advantage that the further alloying components do not have to be admixed in powder form. In particular, oxygen contamination is reduced thereby. The attritor vessel, the grinding balls and/or the grinding arms of the agitator are thus actively used as suppliers of alloying components. When a vacuum is applied to the attritor vessel, protective gas, such as argon or helium, may preferably be used for scavenging purposes, to remove the residual oxygen. Filling of the attritor vessel with the base material powder preferably takes place under a vacuum.

In a further advantageous embodiment of the invention, at least the surface of the grinding balls contains the base material and/or at least one of the alloying components of the target alloy. Alternatively or in combination, at least the internal walls of the attritor vessel may contain the base material and/or at least one of the alloying components which the target alloy comprises. Alternatively or in combination, at least the surface of the grinding arms of the agitator may contain the base material and/or at least one of the alloying components which the target alloy comprises. The components of the attritor (attritor vessel, grinding balls and/or agitator with the grinding arms) may be provided with a coating, which contains the base material and/or at least one of the alloying components. Alternatively or in combination, at least one component of the device for mechanical alloying may consist fully—apart from unavoidable impurities—of the base material and/or at least one of the alloying components. These are preferably the grinding balls and/or the grinding arms of the agitator. The attritor vessel may be lined internally with replaceable tiles, which constitute the internal walls of the attritor vessel. These tiles may in turn consist completely—apart from unavoidable impurities—of the base material and/or of at least one of the alloying components.

In particular the following aspects and combinations thereof are encompassed by the invention:

1. A method for producing a high temperature-resistant target alloy, comprising

-   -   (a) applying a vacuum to an attritor vessel containing the base         material of the target alloy,     -   (b) filling the attritor vessel with a powder containing the         base material of the target alloy with a reduced alloy element         content,     -   (c) filling the attritor vessel with grinding balls containing         the base material of the target alloy,     -   (d) rotating the agitator of the attritor and/or of the attritor         vessel, wherein the powder is alloyed by attrition of the         attritor and/or attritor vessel and the grinding balls         themselves.

2. The method according to item 1, wherein the target alloy contains TiAl.

3. The method according to items 1 or 2, wherein the base material powder is plasma-cleaned prior to filling and/or the attritor vessel is plasma-cleaned prior to application of a vacuum.

4. The method according to any one of the preceding items, wherein mechanical alloying takes place under a vacuum of from about 1×10⁻⁶ to about 1×10⁻⁴ mbar or under an inert protective gas atmosphere, in particular helium or argon, at from about 1×10⁻³ mbar to about 2000 mbar for a period of from about 0.5 h to about 10 h and at a temperature of less than or equal to about 400° C.

5. The method according to any one of the preceding items, wherein

-   -   the powder of the base material in (b) comprises powder grains         with a diameter of less than or equal to about 500 μm and in         particular, with a diameter of at least about 15 μm and/or     -   (d) proceeds at a rotational speed of from about 30 to about         2000 rpm for a period of from about 1 to about 10 hours.

6. The method according to any one of the preceding items, wherein the mechanically alloyed powder of the target alloy is heat-treated in a subsequent step in such a way that fine oxides are eliminated and/or the residual oxygen is gettered out of the crystal lattice of the powder.

7. The method according to item 6, wherein the powder of the target alloy is heat-treated by laser or electron beam melting, laser metal deposition and/or by hot isostatic pressing and the fine oxides with a size of from about 1 nm to about 500 nm are eliminated.

8. The method according to item 7, wherein the hot isostatic pressing proceeds in a temperature range of from about 1000 ° C. to about 1500 ° C. for a period of from about 1 h to about 10 h at a pressure of from about 10 to about 500 MPa.

9. The method according to any one of the preceding items, wherein

as alloying component at least

-   -   one of the elements Si, Y, Hf, Er, Gd, B, C, Zr, Y, Hf, Nb, Mo,         W, Co, Cr, V is present and/or     -   at least one compound from the group tungsten carbide, tungsten         molybdenum alloys, zirconium oxide and yttrium oxide is present

and/or

as main constituent of the target alloy and/or of the powder of the base material at least one of the elements Fe, Ni, Ti, Al, Mo is present.

10. A device for mechanically alloying a high temperature-resistant target alloy, comprising at least the following components:

-   -   an attritor vessel with internal walls,     -   an agitator and     -   at least one grinding ball,

wherein all the components of the device which come into contact with the powder during mechanical alloying contain the base material and/or at least one of the alloying components of the target alloy.

11. The device according to item 10, wherein at least the internal walls of the attritor vessel comprise the base material and/or at least one of the alloying components of the target alloy.

12. The device according to at least one of items 10 and 11, wherein at least the surface of the grinding balls comprises the base material and/or at least one of the alloying components of the target alloy.

13. A high temperature-resistant alloy, produced using a method according to any one of items 1 to 9.

14. The alloy according to item 13, wherein the alloy contains at least one of the elements iron, nickel, titanium, aluminum, molybdenum.

15. Use of a device according to any one of items 10 to 13 in a method according to any one of items 1 to 9 for producing a high temperature-resistant target alloy.

DETAILED DESCRIPTION OF EMBODIMENTS

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description making apparent to those of skill in the art how the several forms of the present invention may be embodied in practice.

EXEMPLARY EMBODIMENT

First of all, an attritor vessel, whose internal walls consist of the base material, for example Ti and/or Al, and of some or all of the alloying components of the target alloy is plasma cleaned at a low pressure of from 0.05 to 200 Pa in an alternating electrical field by ionization of the real gas atoms. Then the attritor vessel is degassed at 10⁻³ mbar at a temperature of T=400° C. for 2 hours.

The base material powder, for example of Ti and Al and for example Cr, V, W, Mo, Fe, Co, Zr, C and/or B, is likewise plasma cleaned under the same conditions and then loaded into the attritor vessel. The attritor accommodates around 5 kg of powder.

The grinding arms, already located in the attritor vessel, of the agitator preferably consist only of Ti, Al and only of the corresponding alloying components, as do the grinding balls. The grinding balls have a diameter of around 2 cm. The grinding arms and the grinding balls are preferably formed from the solid material of an alloy similar or identical to the target alloy, such that not only does the surface of the grinding balls or of the grinding arms consist of the “target alloy” but also the material located under the surface.

An alloy similar to the target alloy means that this similar alloy must not have any alloying components which are not present in the target alloy. The similar alloy may in this case comprise fewer alloying components than the target alloy, wherein the proportions of the alloying components in the similar alloy may be different from the target alloy.

The attritor vessel is filled with grinding balls and then closed. Agitation is performed for 5 hours at a rotational speed of 100 rpm.

To form the oxides, the mechanically alloyed powder with the corresponding alloying components is then hot isostatically pressed at 1200° C. for 3 hours at 2000 bar (200 MPa) in a helium protective gas atmosphere. Hf, Y, Zr, Er and Gd oxides arise in the process, which are finely distributed in the matrix.

For example low pressure turbine (LPT) blades, LPT stators and/or LPT disks may consist of such an alloy. Hot gas baffles and/or further structural elements of a non-stationary or stationary gas turbine may also consist of such a target alloy.

The above method may also be used for alloying other base materials. To this end, the base material of titanium and aluminum may be replaced for example by molybdenum, nickel or iron. The above-described alloying components and proportions may in this respect be identically selected for molybdenum, nickel or iron.

Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 

What is claimed is:
 1. A method for producing a high temperature-resistant target alloy, wherein the method comprises (a) applying a vacuum to an attritor vessel containing a base material of the target alloy, (b) filling the attritor vessel with a powder comprising the base material of the target alloy with a reduced alloy element content, (c) filling the attritor vessel with grinding balls comprising the base material of the target alloy, (d) rotating an agitator of the attritor and/or the attritor vessel, the powder being alloyed by attrition of the attritor and/or the attritor vessel and the grinding balls.
 2. The method of claim 1, wherein the target alloy comprises TiAl.
 3. The method of claim 1, wherein the base material powder is plasma-cleaned prior to (b) and/or the attritor vessel is plasma-cleaned prior to (a).
 4. The method of claim 1, wherein mechanical alloying takes place under a vacuum of from 1×10⁻⁶ to 1×10⁻⁴ mbar or under an inert protective gas atmosphere at from 1×10⁻³ mbar to 2000 mbar for a period of 0.5 h to 10 h and at a temperature of less than or equal to 400° C.
 5. The method of claim 1, wherein the powder of the base material in (b) comprises powder grains with a diameter of less than or equal to 500 μm and/or (d) is carried out at a rotational speed of from 30 to 2000 rpm for a period of from 1 to 10 hours.
 6. The method of claim 1, wherein mechanically alloyed powder of the target alloy is heat-treated in a subsequent step in such a way that fine oxides are eliminated and/or residual oxygen is gettered out of a crystal lattice of the powder.
 7. The method of claim 6, wherein the powder of the target alloy is heat-treated by laser or electron beam melting, laser metal deposition and/or by hot isostatic pressing and fine oxides having a size of from 1 to 500 nm are eliminated.
 8. The method of claim 7, wherein the hot isostatic pressing is carried out at a temperature ranging from 1000° to 1500° for a period of from 1 h to 10 h at a pressure of from 10 to 500 MPa.
 9. The method of claim 1, wherein as alloying component at least one of Si, Y, Hf, Er, Gd, B, C, Zr, Y, Hf, Nb, Mo, W, Co, Cr, V is used and/or at least one compound from the group tungsten carbide, tungsten molybdenum alloys, zirconium oxide and yttrium oxide is used and/or as main constituent of the target alloy and/or of the powder of the base material at least one of Fe, Ni, Ti, Al, Mo is present.
 10. The method of claim 9, wherein as alloying component at least one of Si, Y, Hf, Er, Gd, B, C, Zr, Y, Hf, Nb, Mo, W, Co, Cr, V is used.
 11. The method of claim 9, wherein as alloying component at least one compound from the group tungsten carbide, tungsten molybdenum alloys, zirconium oxide and yttrium oxide is used.
 12. The method of claim 1, wherein a device comprising at least an attritor vessel with internal walls, an agitator, and at least one grinding ball is used and wherein all components of the device which come into contact with powder during mechanical alloying comprise the base material and/or at least one of the alloying components of the target alloy.
 13. The method of claim 1, wherein in addition to main constituents, the powder comprising the base material is formed from, apart from unavoidable impurities: W: 0 to 8 at. %, C: 0 to 0.6 at. %, Zr: 0 to 6 at. %, B: 0 to 0.2 at. %, Nb: 4 to 25 at. %, Mo: 1 to 10 at. %, Co: 0.1 to 10 at. %, Cr: 0.5 to 3 at. % and/or V: 0.5 to 10 at. % and wherein the target alloy, in addition to main constituents, is formed from, apart from unavoidable impurities: W: 0 to 8 at. %, Si:
 0. 2 to 0.35 at. %, C: 0 to 0.6 at. %, Zr: 0 to 6 at. %, Y: 0 to 1.5 at. %, Hf: 0 to 1.5 at. %, Er: 0 to 0.5 at. %, Gd: 0 to 0.5 at. %, B: 0 to 0.2 at. %, Nb: 4 to 25 at. %, Mo: 1 to 10 at. %, Co: 0.1 to 10 at. %, Cr: 0.5 to 3 at. % and/or V: 0.5 to 10 at. %.
 14. The method of claim 13, wherein the main constituents comprise Ti and Al.
 15. A device for mechanically alloying a high temperature-resistant target alloy according to the method of claim 1, wherein the device comprises at least an attritor vessel with internal walls, an agitator, and at least one grinding ball, all components of the device coming into contact with powder during mechanical alloying comprising the base material and/or at least one of the alloying components of the target alloy.
 16. The device of claim 15, wherein at least internal walls of the attritor vessel comprise the base material and/or at least one of the alloying components of the target alloy.
 17. The device of claim 16, wherein at least a surface of the grinding balls comprises the base material and/or at least one of the alloying components of the target alloy.
 18. A high temperature-resistant alloy, wherein the alloy is obtained by the method of claim
 1. 19. The alloy of claim 18, wherein the alloy comprises at least one of iron, nickel, titanium, aluminum, molybdenum.
 20. The alloy of claim 18, wherein the alloy comprises Ti and Al as main constituents. 